The extracellular matrix of healthy and atretic mouse ovarian follicles studied by lanthanum nitrate and polycation ruthenium red

Changes occurring in the fine structure of Zona pellucida and follicular fluid in growing and atretic follicles in mice were examined by electron microscopy using lanthanum nitrate and ruthenium red. The results showed that follicular fluid has a dense globular aspect in growing follicles but becomes more dispersed and globular-fibrillar in atretic follicles. It is formed by rounded negatively charged particles (20 nm in diameter) that are closely packed in growing follicles, but spaced and united by filaments in atretic follicles. These changes may be related to a changed role of granulosa cells and also to an altered passage of nutritive substances from the plasma into the follicle. The Zona pellucida is intensely colorable only in the atretic follicles, where it is composed of a dense globular fibrous matrix, but not in growing follicles, where it reveals a weak reaction with ruthenium red. While in the Zona pellucida of growing follicles the globular negatively charged particles (20 nm in diameter) probably due to sialic acid moieties are rarely evident, these particles become highly numerous and connected by microfilamentous structures in atretic follicles. The altered electrical charge of polyanions may reflect a different chemical permeability of the Zona pellucida. Then the increase of negative charged particles in the zona may have a role in a process of sequestration of the oocyte during atreasia.